The publication, citing documents it obtained under a public records act request, said Apple has met with officials from the GoMentum Station, a large former Navy weapons station near San Francisco that is being changed into a high-security testing area for self-driving cars. GoMentum says its 5,000-acre facility, which features 20 miles of paved roadway, "is the largest secure test facility in the world and will become the center" of connected vehicle applications and autonomous vehicles technologies -- something that could appeal to a secretive company like Apple. Honda uses the facility to test automated driving technologies.

Frank Fearon, an Apple engineer, wrote to GoMentum that "We would... like to get an understanding of timing and availability for the space, and how we would need to coordinate around other parties who would be using [it]," according to the publication. In another email from a GoMentum official to Fearon, the official delayed a tour of the facility but said, "We would still like to meet in order to keep everything moving and to meet your testing schedule."

The Guardian didn't publish the documents cited in the report, and it's unclear whether Apple directly said it's building a self-driving electric car. The company could be interested in the facility for other purposes, such as testing out car technologies in a more real-to-life environment instead of in a lab. And there's no guarantee that Apple will release an electric self-driving car even if it's currently researching the technology.

Autonomous car technology has become a big focus for companies such as Google and Uber, and speculation about Apple's self-driving car plans have been swirling for months. The program is believed to be codenamed "Titan" and involve hundreds of engineers. The company has hired people from the automotive industry, including battery experts. In February, A123 Systems, an electric-car battery maker, sued Apple for poaching its employees, saying the company lured away workers to develop "a large-scale battery division to compete in the very same field as A123." The two companies reached a settlement in May.

Apple has tasked employees in "an anonymous office building" in Sunnyvale, Calif., about four miles from the company's Cupertino headquarters, with developing automotive technologies, the Guardian said. The company leased the building in 2014, the Guardian said, citing documents, and modified the facility to include labs and workshop spaces, as well as tighter security features.

Apple, the second-biggest smartphone maker in the world, has worked to expand its technologies to many different sectors and become the center of peoples' lives. That already has included cars, even if the company hasn't created a full-blown automobile. An update to its iOS mobile software in March 2014 incorporated CarPlay -- a way for the iPhone to power a touch screen on a new car's dashboard. And Jeff Williams, Apple's head of operations, in May called the car "the ultimate mobile device."

Marc Newsom, a designer who has worked with Apple in the past, told The Wall Street Journal earlier this week that his design pet-peeve is the automotive industry. "There were moments when cars somehow encapsulated everything that was good about progress," he said. "But right now we're at the bottom of a trough."